1. Field of the Invention
The invention relates to a base paper for waterproof coated paper carriers for photographic layers as well as a method for the production of such a base paper for the production of layer carriers.
2. Brief Description of the Background of the Invention Including Prior Art
It is known to employ paper laminate coated on two sides with hydrophobic resin layers as carrier material for photographic layers to be developed with a wet process. Such a carrier material comprises in a widely used embodiment a base paper with polyolefin resin coatings applied to both surfaces. The coating disposed next to the photographic layers contains usually at least one light-reflecting pigment such as, for example, titanium dioxide.
A paper provided on the two surfaces with a water-impermeable resin layer is protected on its faces against an entering and penetrating of water or of aqueous photographic processing solutions, but it is not protected at the open edges, where the paper core of the laminate is freely exposed. Aqueous processing solutions can therefore penetrate at the edges of the laminate into the paper core and cannot be removed without residue by the usual wash processes.
In order to prevent or, respectively, to decrease this penetration of acqueous solutions at the edges of the laminate, the base paper is usually hard sized. Both, reactive materials, such as for example dimerized alkylketene, as well as non-reactive materials, such as higher fatty acids, are known as sizing agents for photographic base paper (Journal of Applied Photographic Engineering 7, 1981, pp. 67-72). Reactive sizing agents are usually processed in the presence of a cationic resin without a pH-change, i.e. "neutral". Non-reactive sizing agents require for fixing an addition of aluminum ions and are processed in acid solutions. The U.S. Pat. No. 4,659,430 teaches in addition a combination of these two sizing processes in order to protect the paper core still more against the penetration of the various components of photographic processing solutions.
The sizing agents, acting to impart hydrophobic properties, are usually added to the fiber suspensions and are deposited by addition of auxiliary agents on the surface of the fibers. Such an auxiliary agent is, for example, an aluminum salt in the case of an acid sizing and, in case of a neutral sizing, for example, a cationic polyamide-polyamine-epichlorohydrine resin, which is employed predominantly together with C.sub.16 -C.sub.18 -alkyl-ketene dimer (Journal Applied Photographic Engineering 7, 1981, pp. 67-72). However, also other cationic materials, such as cationic polyacryl amides, cationized starches, or polyethylene-imines, are suitable in certain cases for the improvement of the retention and for the fixation of reactive sizing agents in the paper sheet.
Since the sizing agents are deposited at the surface of the paper-making pulp fibers, naturally, fiber-fiber-bonding becomes weaker in a sized paper as compared to a non-sized paper. Consequently, additional water-soluble resins are widely employed in order to improve the paper strength. Such resins include for example polyvinyl-alcohol, anionic or cationic polyacryl-amide, guar resin or gum, and various starch derivatives. Guar resin or guar gum is a water-soluble mucilage obtained from Cyanopsis tetragonoloba with the water-soluble portion of the guar flour containing about 35% galactose, 63% mannose, and 5 to 7% protein.
U.S. Pat. No. 4,665,014 describes for example the use of anionic polyacryl amide together with cationic starch. U.S. Pat. No. 4,433,030 teaches the additional use of cationic polyacryl amides and U.S. Pat. No. 4,439,496 teaches the use of anionic polyacryl amide together with cationic polyacryl amide, whereas U.S. Pat. No. 4,675,245 describes a combination of cationic resin with anionic polysaccharide derivatives. The addition of the strengthening resins serves at the same time the purpose to decrease the discoloring of the edges by the developer.
However, not only the loss of strength is disadvantageous in a hard sizing of the base paper, where the decrease in the structural strength is particularly damaging in case of laminates. In particular, it is disadvantageous that the use of flocculating auxiliaries influences disadvantageously the sheet formation. This is based on the fact that the addition of cationic materials not only retains the sizing agent at the fiber surface but, in addition, unavoidably and despite counteracting machine-technical measures, fiber flocs form at an increased extent, because the repulsive negative charges of the fibers are substantially neutralized by the cationic materials and fiber-fiber-bridge formations are facilitated. Depending on the materials used and the machine-technical apparatus, the fiber flocs have various sizes and distribution in the paper sheet, and they determine the homogeneity and the surface quality of the base paper. The structure, once formed in the paper sheet, can no longer be changed even by intensive calendering.
The described combined use of cationic and anionic water-soluble resins achieves in fact a decrease of the bath penetration at the cutting edges, as described in the U.S. Pat. No. 4,665,014, U.S. Pat. No. 4,439,496, U.S. Pat. No. 4,433,030, or the U.S. Pat. No. 4,675,245. However, the problem of the sheet formation, the surface quality, and the structural strength have not been considered in these references. Only in the U.S. patent application, Ser. No. 019,580, have these two points been considered in, but only in connection with an acid sizing.
The combined use of anionic and cationic resins regularly results in inferior the sheet formation in neutral sized papers. In addition, the structural and textural strength of the paper sheet decreases frequently because, in case of an unstable control of the process, isolated flocs of anionic and cationic resin are generated, which are deposited and included into the paper web without generating a strengthening effect.
In order to avoid these flocculation problems, it has been attempted to add to the cellulose suspension non-ionic polymers such as, for example, starch, polyvinyl alcohol, or guar gum. In fact, this resulted in a certain improvement in the sheet formation and the surface quality, however, the retention of these materials was substantially worse, the production speed had to be reduced and, after treatment with the photographic processing solutions, the internal strength turned out to be substantially decreased. The edges of the coated layer carriers were split under conditions of photographic processing solution.
Hitherto, according to the U.S. patent application, Ser. No. 019,580, a good sheet formation and a sufficient surface quality could be achieved with some certainty only with acid sized papers. However, in many cases, an acid sizing is not desirable because the sensitivity of photographic layers can thereby be decreased.
However, even papers according to the U.S. patent application, Ser. No. 019,580, can be improved. It is disadvantageous that, in case of the use of two differently charged polyacryl amides, two mixing stations and two metering stations are necessary. It is further disadvantageous that the metering of the two components cannot be precisely coordinated with respect to each other because, on the one hand, the electrochemical character of the two polyacryl amide types in solution is not stable and, on the other hand, the electrochemical behavior of the fibers is subject to continuous variations. It results therefrom that the internal strength and structural strength and the sheet structure are subject to variations, which so far have been neither reliably explained nor controlled.
The disadvantage of the state of the art described above is that none of the so far described methods is suitable both for "neutral" sized papers with reactive sizing agents as well as for "acid" sized papers with non-reactive sizing agents. For example, in an attempt to transfer, for example, the teaching of the U.S. patent application, Ser. No. 019,580, to a neutral sized paper, all the disadvantages of the teachings of the U.S. Pat. No. 4,439,496 reappear. The described, unexplainable variations in the structural strength, the sheet structure, and the edge penetration, together with bad sheet formation and bad surface quality.